The rational development of new antineoplastic agents directed against tubulin, a protein critical for cell division, requires greater understanding of the interaction between the polypeptide subunits of tubulin, its two tightly bound guanine nucleotides, and microtubule- associated proteins (MAPs). The effects of nucleotides on the stability of microtubules continued to be examined, as were conditions to optimize the separation of alpha-tubulin and beta-tubulin on a preparative scale. The purification of a microtubule-associated protein which causes the formation of microtubule bundles continued to progress, and a project to introduce potentially antimitotic nucleotide analogs into cells continued. The roles of Mg2+ in the binding of GTP at the exchangeable site, in microtubule assembly, and in microtubule stability were examined in detail. We have succeeded in synthesizing 2',3'-dideoxyguanosine 5'- [alpha, beta-methylene]-triphosphate, and we are evaluating its effects on microtubule assembly.